Circuit board test systems are used to test large numbers of printed circuit boards by verifying electrical functionality and continuity between various test points on the circuit board. Continuity and lack of continuity are generally detected through the use of test fixtures having an array of test probes for making electrical contact with individual test points on the printed circuit board. Test systems vary in their approach to bringing the array of test probes into contact with the test points on the circuit board. One test system for testing loaded circuit boards includes spring-biased test probes mounted on a probe plate in a vacuum-actuated test fixture for applying pressure between the spring probes and the circuit board test points. The test system also includes a test analyzer which generally includes a computer controlled detection system for applying electrical test signals to selected contacts to sense continuity and provide test results. An example of a prior art test system is described in Long, et al. U.S. Pat. No. 4,138,186.
Loaded printed circuit boards are usually crowded with various electrical components including arrays of integrated circuit packages. These packages typically include integrated circuits packaged in an insulated housing with a plurality of thin, parallel electrical leads extending from the housing. The leads from the integrated circuit package can be bent into various configurations which can make testing of the packages difficult. For instance, integrated circuit packages with "J-lead" or "gull wing" lead configurations are common. It is also common to mount large numbers of these integrated circuit packages close to one another on the circuit board to save board space. If a number of integrated circuit packages are crowded together on a loaded circuit board, there is usually little room left around the integrated circuit packages for making electrical contact between spring probes on a test unit and leads on the circuit packages or other circuit elements adjacent to the packages.
Semiconductor chips are often produced in various packages. One type of package is a dual-in-line (DIP) package having two sets of spaced apart leads extending along opposite sides of the package. A second method of packaging uses surface mount packaging technology which packs electronic functions more densely on circuit boards. One common form of surface mount packaging is the so-called plastic leaded chip-carrier (PLCC) package. Generally speaking, PLCC packages house memory and microprocessor integrated circuit chips requiring large numbers of leads spaced apart along all four sides, or at least along two opposite sides of a rectangularly shaped housing. Because of their small size and large number of leads, it is difficult to test multiple arrays of closely spaced apart PLCC devices mounted on loaded printed circuit boards. The configuration of the leads also makes it difficult to test PLCC devices. In some integrated circuit packages, the lead configuration prevents contact being made between the leads and the test probes of the test unit, for instance, the packages with leads in a gull wing configuration. The solder joints are sensitive to external pressure and therefore contact with spring biased test probes should be avoided to prevent fracturing the solder joints.
Translator modules for testing such integrated circuit packages are described in Van Loan, et al. U.S. Pat. 5,049,813 (the '813 patent herein), the subject matter of which is incorporated herein by this reference. The '813 patent shows a translator module mounted over an integrated circuit package on a loaded circuit board. The translator module includes rows of spaced apart contacts for making contact with corresponding rows of leads adjacent a circuit package. In one embodiment, an array of test pads on a top side of a module are contacted by individual test probes of a circuit continuity test analyzer. Electrical contact between the leads associated with the circuit package and corresponding contacts on the module is translated to the test probes for use in verifying electrical connections between the circuit package and circuit elements on the printed circuit board.
In one embodiment of the '813 patent, the contacts on the test housing are arranged for independent releasable spring biased contact with corresponding leads on the integrated circuit package. In another embodiment, the module is made from a "flex-circuit" material so that the contacts each comprise a thin metal film on a flexible plastic backing sheet, in which the contacts have a memory for producing rows of spring-like contact fingers along the side edges of the module. In other forms of the test module, the spring probes or flex circuit fingers may be arranged to contact corresponding test points or pads on the printed circuit board adjacent the leads on the IC package.
As circuit board and integrated circuit technologies advance, circuit components are more closely spaced. The test points and probes for such circuits necessarily must be more closely spaced, requiring smaller probes and smaller probe components such as springs which become more difficult and expensive to make.
Thus, there is a need to provide a means for testing loaded printed circuit boards having large numbers of integrated circuit packages mounted close to one another on the board where the circuit packages have leads closely spaced to one another. The test system must be adaptable to the closer spacings while providing extremely accurate and reliable test measurements. The need for such a test system is especially critical because of the expanding use of the surface mount packages such as PLCC devices which are particularly difficult to test. PLCC devices in particular can be difficult to test with a conventional test head because the PLCC package can easily become skewed from a squared position relative to the circuit board, or because of the need to avoid pressure contact between the leads on the IC package and the test unit. There is also a need for a test system that does not cause undue delays during testing and is adaptable readily to testing circuit boards having multiple integrated circuit packages mounted in various patterns on different boards. In addition, the test system should be reasonably inexpensive to manufacture and highly reliable during use.